The so-called GC (Garbage Collector) roots are objects special for garbage collector.
Garbage collector collects those objects that are not GC roots and are not accessible by references from GC roots.

There are several kinds of GC roots. One object can belong to more than one kind of root.
The root kinds are:

Class - class loaded by system class loader. Such classes can never be unloaded.
They can hold objects via static fields.
Please note that classes loaded by custom class loaders are not roots, unless
corresponding instances of java.lang.Class happen to be roots of other kind(s).

Thread - live thread

Stack Local - local variable or parameter of Java method

JNI Local - local variable or parameter of JNI method

JNI Global - global JNI reference

Monitor Used - objects used as a monitor for synchronization

Held by JVM - objects held from garbage collection by JVM for its purposes.
Actually the list of such objects depends on JVM implementation.
Possible known cases are: the system class loader, a few important exception classes which the JVM knows
about, a few pre-allocated objects for exception handling, and custom class loaders when they are
in the process of loading classes.
Unfortunately, JVM provides absolutely no additional detail for such objects.
Thus it is up to the analyst to decide to which case a certain "Held by JVM" belongs.

If an object is a root, it is specially marked in all views showing individual objects.
For example, the following picture shows a fragment of paths view: